The present invention concerns a device for winding a barrel spring, for example of a timepiece, using the energy of ambient light falling on a solar cell.
Such devices are well known. In one construction they comprise a solar cell receiving ambient light, and a continuously rotating electric motor connected to the cell and coupled to the spring possibly via a gear train. For the motor to be able to turn and wind the spring, the intensity of the ambient light must exceed a given threshold which depends on the resisting torque of the barrel spring. Below this threshold, the light energy falling on the cell is lost.
This drawback is eliminated in a second construction described in Swiss patent CH 428 576 or its equivalent GB 904 400. Here, the device further comprises a capacitor connected to the terminals of the cell and switching relay-means enabling connection of the motor to the terminals of the capacitor when the voltage thereof exceeds a reference voltage corresponding to the voltage necessary for starting rotation of the motor. Since the cell acts essentially like a current generator with the magnitude of the current depending on the light energy received, it is able, even with poor lighting, to charge progressively the capacitor when it is not connected to the motor. Once the reference voltage is reached, the motor is connected by the switching relay-means to the capacitor which thus supplies it for a given time interval with sufficient energy to rewind the spring. After this period of time, when the voltage of the capacitor has become too low to sustain rotation of the motor, the motor is disconnected. Another cycle of charging of the capacitor by the cell and discharge to the motor may then begin.
The ratio between the charging and discharging times of the capacitor depends of course on the intensity of the ambient light illuminating the cell. For a low light intensity, the ratio may be large and in this case the motor operates intermittently. For high light intensities, the ratio may be zero so the motor rotates continuously.
The second type of winding device is an improvement over devices where the motor is directly connected to the cell, but nevertheless still has some drawbacks. On the one hand, the period of time during which the motor is connected to the capacitor is determined in an imprecise manner by mechanical characteristics of the relay forming the switching means. The ill-defined value of this time period means that it cannot correspond to the optimum time for which the device provides the most efficient conversion of light energy into mechanical energy. On the other hand, because with strong illumination the voltage at the terminals of the capacitor (and hence of the motor) remains constant, this device can only be associated with commutator motors. Such motors do not lend themselves to the high degree of miniaturisation necessitated for certain applications, such as the winding of the barrel spring of a watch for example.
The object of the present invention is to propose a winding device which does not suffer from these drawbacks.
To achieve this, the barrel spring winding device according to the invention comprises:
a capacitor;
a solar cell arranged to receive ambient light and to charge the capacitor;
a control circuit connected to terminals of the capacitor; and
a motor connected to the circuit and having a rotor operatively connected to wind the spring,
and is particularly characterized in that the motor is a stepping motor having two terminals, and that the control circuit comprises:
means for supplying a drive signal having two states, a first state produced in response to a parameter representing the state of charging of the capacitor, and a second state produced after a given period of time from the beginning of the first state; and
means for connecting the motor to the terminals of the capacitor in response to the first state of the drive signal and for disconnecting the motor from the capacitor in response to the second state of the drive signal.
One advantage of the device according to the invention is that the motor receives pulses of precisely defined duration and amplitude which ensures optimal operating conditions for the device.
Another advantage of the device is that it comprises a stepping motor, i.e. the type of motor whose miniaturization involves the fewest problems. These stepping motors are the only ones that can be used where the available space is extremely restricted, as is in particular the case with watches.
Other characteristics and advantages of the winding device according to the present invention will be apparent from the following description, made with reference to the accompanying drawings and given by way of non-limiting explanation, of an embodiment of such a device.